A crystalline silicon-based solar cell including a conductive silicon-based thin-film on a single-crystalline silicon substrate is called a heterojunction solar cell. Particularly, a heterojunction solar cell including an intrinsic amorphous silicon thin-film between a conductive silicon-based thin-film and a crystalline silicon substrate is known as one of forms of crystalline silicon-based solar cells having the highest conversion efficiency.
A heterojunction solar cell includes silicon-based thin-film of opposite-conductivity-type on the light-receiving side of a crystalline silicon substrate of first conductivity-type, and silicon-based thin-film of first conductivity-type on the back side of the crystalline silicon substrate of first conductivity-type. Generally, an n-type single-crystalline silicon substrate is used, and a p-type silicon-based thin-film is formed on the light-receiving side thereof, while an n-type silicon-based thin-film is formed on the back side thereof. Carriers generated at these semiconductor junction portions are extracted outside of a solar cell via an electrode. As the electrode, a combination of a transparent electroconductive layer and a metal collecting electrode is generally used.
Since the metal collecting electrode shields light, a line-shape patterned metal collecting electrode is used on the light-receiving side for enlarging the light-receiving area of the solar cell. On the other hand, an attempt has been made to improve light utilization efficiency by a method in which a metal electrode is formed on the entire surface on the back side, and light that has been transmitted without being absorbed by a crystalline silicon substrate is reflected by the metal electrode on the back side to reenter into the crystalline silicon substrate. For example, Patent Document 1 discloses a heterojunction solar cell in which a patterned collecting electrode is formed on the light-receiving side of the solar cell by a plating method, and a silver electrode is formed on the entire surface on the back side of the solar cell by a sputtering method. Patent Document 2 discloses a heterojunction solar cell in which a metal electrode is formed on the entire surface on the back side by electroplating. In electroplating, a metal electrode having a large thickness can be easily formed, and therefore improvement of characteristics and productivity by reduction of the resistance of the metal electrode can be expected.